1. Field of the Invention
The present invention relates to a monolithic tactile sensor and, more particularly, to a monolithic tactile sensor which combines considerable spatial resolution with pressure sensing over a range of two orders of magnitude.
2. Description of the Prior Art
The ability of a robot to pick up and manipulate mechanical parts can be greatly improved if the grasping surfaces are able to sense pressure. This type of sensor can be used for object identification as well as for insertion and mating tasks associated with assembly. The preferred location of the sensor is on the extremity of a movable finger. Consequently, the sensor must be compact and rugged and should provide spatial resolution of better than 0.1 inch .sup.2 /element (one hundred sensing elements per square inch) to be compatible with industrial applications. While some progress has been made in developing tactile (touch) sensors, these sensors are crude in comparison to the available image and audio transducers. Two general methods are used for tactile sensing: force and torque measurements at either the manipulator wrist or at the work piece, and direct "finger-tip" pressure sensing.
One type of force sensor is disclosed in U.S. Pat. No. 4,286,459 issued to W. S. N. Trimmer et al on Sept. 1, 1981. As disclosed, the Trimmer et al force sensor includes at least one strip of flexible piezoelectric material having an electrically conductive coating on opposite sides thereof. When a force is applied to the piezoelectric strip causing it to stretch, the strip will oscillate at a frequency that is determined by the magnitude of the applied force. Accordingly, the magnitude of the applied force can be determined by measuring the frequency of the resulting oscillation.
U.S. Pat. No. 4,306,148 issued to C. G. Ringwall et al on Dec. 15, 1981 discloses an alternative tactile sensor. The Ringwall et al arrangement relates to a tactile area sensor for robots which has an array of pneumatic flow passages. The air flow in each passage is dependent on a localized force excited by an object pressing against an elastic pad on the face of the sensor. The air flow impinges on a metallic tab and its angular displacement is sensed by directing a light beam from an optical fiber onto the tab and monitoring the quantity of light reflected to a paired optical fiber.
An ultrasonic transducer array and imaging system is disclosed in U.S. Pat. No. 3,979,711 issued to M. G. Maginness et al on Sept. 7, 1976. The array comprises a plurality of transducer elements each having a major face capable of bidirectionally transmitting and receiving ultrasonic energy. The opposite face of each element comprises means for bidirectionally providing and receiving electrical energy across each transducer element. In the imaging system, the array may be selectively scanned to provide a fine detail structure image over an extended area.
The provision of "finger-tip" tactile sensing for application to the real time sensory feedback control of mechanical manipulators remains largely an unsolved problem. Progress in this area has been impeded primarily by the lack of an adequate transducer. Factors such as nonlinearity, hysteresis and limited dynamic range have for the most part limited tactile sensors to a simple binary response. The problem remaining in the prior art, therefore, is to provide a "finger-tip" tactile sensor with more than a simple contact/no contact response.